Current demands for high density and performance associated with ultra large scale integration require submicron features, increased transistor and circuit speeds and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring, including frequent and detailed inspections of the devices while they are still in the form of semiconductor wafers.
Conventional in-process monitoring techniques employ a two phase “inspection and review” procedure. During the first phase the surface of the wafer is inspected at high-speed and relatively low-resolution. The purpose of the first phase is to produce a defect map showing suspected locations on the wafer having a high probability of a defect. During the second phase the suspected locations are more thoroughly analyses. Both phases may be implemented by the same device, but this is not necessary.
The two phase inspection tool may have a single detector or multiple detectors. A multiple detector two phase inspection device is described in U.S. Pat. Nos. 5,699,447, 5,982,921 and 6,178,257B1 of Alumot (hereinafter collectively referred to as the Alumot system) whose contents are hereby incorporated herein by reference.
During the first phase (also referred to as the inspection phase) the Alumot system (a) obtains an inspected pixel, neighboring inspected pixels and a reference pixel, (b) determines the type of the inspected pixel and/or determined the reference pixel type, (c) compares between the inspected pixel and the reference pixel and a threshold that depends upon the inspected pixel type, (d) and determines a presence of a defect in response to said comparison. The step of determining the type of a pixel involves a first stage of determining the following parameters: (i) local maxima—whether the pixel is a local maxima (if the pixel is a maximum relative to his neighbors), (ii) intensity—if the pixel is intense (if the intensity of the pixel is significant relative to a threshold), (iii) ratio—what is the ratio between the intensity of a pixel and the intensity of its neighbors relative to a threshold) and (iv) gradient—whether the pixel is located in a slope area relative to a threshold. The second stage involves classifying the pixel to one of the following types, in response to the parameters: (I) isolated peak (if the pixel is a local maxima with significant intensity and ratio), (II) multipeak (if the pixel is not an isolated peak, it has significant intensity and none of its neighbors is an isolated peak), (III) slope—if either one of the pixel's neighbors is an isolated peak or has significant gradient, or (IV) background—if the pixel has no significant intensity or gradient and none of its neighbors is an isolated peak.
Accordingly, the defect detection is responsive to the typing of the inspected pixel as the thresholds are responsive to the type of the pixel. Various errors in the measurement process, especially in noisy environment, and/or measurement inaccuracies may effect the type determination and may result in erroneous defect detections. Furthermore, the set of a pixel's types are fixed. Thus the types cannot be dynamically adjusted or tailored according to an end-user requirement.
The reference pixel is usually obtained from a previous inspection of another die, either from the same wafer or not. A comparison between pixels that were obtained from different dies, especially when the inspection tool parameters could change, may also result in erroneous determinations.
There exists a need for an improved and more robust method for inspecting a substrate, and especially a semiconductor substrate, for defects.
There exists a further need for a method for inspecting a substrate that allows a dynamic definition of pixel types.